Two methods for the production of aliphatic or aromatic bridged biscitraconimides are known. The first method is described in Hartford, S. L., et al., J. Polym. Sci. Polym. Chem. Ed., 16 (1982) 137.
This method begins with itaconic anhydride and an aromatic bis-amine. In a two step reaction the bis-amic acid is ring-closed with sodium acetate and acetic anhydride. The biscitraconimides were formed in low yield of less than 50%, as by-products and were purified by column chromotography.
The second synthesis, found in Galanti, A. V., et al., J. Polym. Sci., Polym. Chem. Ed., 19 (1981) 451 and Galanti, A. V. and Scola, D. A., J. Polym. Sci, Polym. Chem. Ed., 20 (1982) 233, is a one-step reaction from itaconic anhydride to aliphatic bridged biscitraconimides. The itaconic anhydride is reacted with an aliphatic bisamine in toluene to form bis-amic acid, which, under reflux and with azeotropic distillation, forms the imide. The compounds are purified by column chromotography followed by recrystallization. The reaction time is 10-16 hours and the yields are between 17 and 70%.
Both of these known synthesis methods suffer from the problems of poor yields and the requirement of a difficult purification process to isolate the biscitraconimide from the reaction products. Further, the first method is performed under difficult reaction conditions, and the second method takes too long. Thus, there is a need in the art for a better synthesis method for making citraconimides.
In maleimide synthesis, the most frequently employed method is a base-catalyzed synthesis employing either sodium acetate or triethylamine as the base catalyst. These methods are described in U.S. Pat. No. 2,444,536 and U.S. Pat. No. 4,130,564, respectively. One synthesis employing sulfuric acid and an onium salt is disclosed in Japanese patent application J6 2072-663.
However, the maleimide synthesis processes cannot be readily adapted to citraconimide synthesis because, during citraconimide synthesis one is always faced with the problem of isomerization of citraconic molecules to itaconic molecules, a problem which does not exist in maleimide synthesis. This isomerization can lead to low selectivity and poor yields.
Finally, citraconic anhydride is a necessary reactant in the present process. The citraconic anhydride may be obtained commercially, or it may be made by the process described in U.S. Pat. No. 2,966,498, or in Galanti, M. C., Galanti, A. N., J. Org. Chem., 47, 1575 (1982).
However, the foregoing process for making citraconic anhydride suffers from the serious disadvantage that water must be completely removed during the synthesis, by distillation. Thus, the time required to melt the itaconic acid starting material and to completely remove the water is too long for the process to be useful on a commercial scale since, citraconimide polymers begin to form during the distillation. Further, the separated water also gave rise to the formation of citraconic acid, an undesirable by-product.
Thus, there is also a need in the art for a better process for the synthesis of citraconic anhydride, which process provides high yields of the citraconic anhydride without significant polymer formation and allows for simple and economically efficient purification of the citraconic anhydride.
These and other objects of the invention will be apparent to one of ordinary skill in the art from the Summary and Detailed Description which follow.